Halogenated phenols are generally useful fungicides, monomers and flame retardants. Certain of the halogenated phenols and their derivatives, for example, brominated aromatic epoxy thermosets, are especially useful in the electronics industry.
However, bromination of most phenols and also certain novolacs is known to provide ring bromo-substitution at positions ortho or para to the phenolic hydroxyl group. See, for example, Jouannetaud et al., U.S. Pat. No. 4,447,660 (1984) and also Japan Kokai No. 60-210615 (85/210615).
Certain meta-halogenated phenols, for example, meta-brominated phenols, are generally known to be more thermally stable than their ortho-halogenated counterparts. See, for example, copending U.S. patent application Ser. No. 851,996, filed Apr. 14, 1986 (incorporated herein by reference). Unfortunately, only a few meta-brominated bisphenols have been described in the literature. K. Auwers & H. Allendorf, Ann., 302, 76-98 (1898), reported the preparation of 2,2', 6,6'-tetrabromo-3,3',5,5'-tetramethyl-4,4'-dihydroxystilbene from 4-bromomethyl-3,5-dibromo-2,6-dimethylphenol. However, this solid is extremely insoluble, which limits its utility.
A more soluble solid is 2,2',6,6'-tetrabromo-3,3',5,5'-tetramethyl-4,4'-dihydroxydiphenylmethane, reported by Auwers et al., Ann., 344, 95-141 (1906) and Ann., 356, 124-51 (1907). It is reported to have been prepared by disproportionation in base of 4-aminomethyl-3,5-dibromo-2,6-dimethylphenol.
More recently, a number of meta-brominated biphenols have been prepared by the bromination of tetraalkyldiphenoquinones. See, for example, Orlando et al., U.S. Pat. Nos. 3,929,908 (1975) and 3,956,403 (1976) and Kinson et al., U.S. Pat. No. 4,058,570 (1977). Using this approach, bromination is the last step in the reaction sequence, and yields are only modest.
In the art of rendering flame-retardant otherwise more flammable materials, for example, thermoplastic polymeric substances such as polystyrenes, concerns include flame-retardant efficiency, formulation simplicity, ease of processability, retention of favorable structural properties and discoloration effects. Flame-retardant efficiency is typically related to the weight ratio of the flame-retardant rendering moiety such as, for example, weight of halogen of an organic halide, to the otherwise more flammable material. Flame-retardant efficiency can be decreased by ongoing processes such as, for example, oozing of the flame retardant. Typically, large amounts of the flame retardant may be thus added to increase the flame-retardant efficiency. In addition, typically large amounts of a flame-retardant synergist such as, for example, an antimony oxide, may be added to increase the flame-retardant efficiency of the flame retardant, for example, the organic halides. A typical formulation to obtain a UL 94 V-O rating may require about 12 weight percent bromine in an aromatic bromide and about 3.5 weight percent antimony oxide (Sb.sub.2 O.sub.3). Moreover, the flame retardant itself may be difficultly processable into, or more importantly, adversely affect the structural properties of, the otherwise more flammable material, for example, the thermoplastics. See, for example, Great Britain Patent Specification No. 1,356,508 (1974). In addition, discoloration effects, for example, so-called "scorch" in a polyurethane, especially in flexible polyurethane foams, may be encountered in commercial production and are undesirable.
In the art of epoxy thermosets, for example, such as in electrical encapsulations and laminates, certain properties of the thermosets are desirable. For example, a higher glass transitoion temperature (T.sub.g) coupled with higher hydrolytic stability such as measured by a low hydrolyzable halide content and with high flame-retardant efficiency is desirable in electrical encapsulations. However, presently available commercial electrical encapsulation systems derived from cresol epoxy novolacs and bis(4-(2,3-epoxypropoxy)-3,5-dibromophenyl)isopropylidene with bromo moieties ortho to the 4-oxy moiety, although of high flame-retardant efficiency (UL 94 V-O rating) and good thermal stability (T.sub.g 155.degree. C., by T.M.A., Thermo Mechanical Analyzer), are hydrolytically unstable (hydrolyzable bromide 180 parts per million (ppm) as measured by total hydrolyzable halide method herein). For example, this hydrolytic instability, even when coupled with the moderately high thermal stability, is problematical because when coupled with moisture penetration, can result in internal corrosion of the encapsulated device, thereby reducing or destroying the effectiveness especially of microelectric circuitry. The moderately high thermal stability itself is also a property needing improvement. A major reason for this is the miniaturization in the art of so-called "microchip" technology. As the scale of these microchips becomes increasingly smaller, the localized heat problems become increasingly greater, and thus, the present encapsulation formulation properties may not be as suitable as desired. In the art of electrical laminates, many of these same considerations apply.